This invention relates to the fabrication of periodic semiconductor structures by molecular beam epitaxy (MBE).
In recent years considerable attention has been given to the proposal of Esaki and Tsu (IBM J. Res. Develop., Vol. 14, p. 61, 1970) that new quantum-mechanical effects in electrical properties can be realized in a structure having a one-dimensional periodic potential ("superlattice") formed by a periodic variation of alloy composition or of impurity density with a period of the order of a few hundred Angstroms (i.e., less than the electron mean free path). The experimental work of Ludeke et al typifies the state of the art of this relatively new technology. They report (Appl. Phys. Let, Vol 24, No. 9, 1974) the use of MBE to fabricate a 161/2 period superlattice in which each period consists of a 50 Angstrom layer of GaAs and a 50 Angstrom layer of Ga.sub.0.75 Al.sub.0.25 As on (100) GaAs substrates held at a temperature between 520 and 600 degrees C. during deposition.
Quantum-mechanical effects in such superlattices have been investigated by Esaki et al in earlier work, tunneling experiments, in which negative resistance was observed. More recently Dingle and Henry (U.S. Pat. No. 3,982,207 issued on Sept. 21, 1976) have proposed a tunable, low threshold double heterostructure (DH) laser in which the active region is a modified superlattice.
What has not been done heretofore, however, is to fabricate a superlattice in which each half period is made up of as little as a single monolayer; that is, a layer which has a thickness equal to a single layer of the compound or element making up the layer. In the GaAs (or AlAs) system, for example, such a monolayer is about 2.8 Angstroms thick and comprises a sheet of Ga (or Al) atoms 1.4 Angstroms thick contiguous with a sheet of As atoms, also 1.4 Angstroms thick. On the other hand, for Ge, a monolayer is a single sheet of Ge atoms about 1.4 Angstroms thick. In fact, the state of the art to date is typified by a paper by A. Y. Cho (Appl. Phy. Let, Vol. 19, p. 467, 1971) which discloses the fabrication of a periodic structure of alternating layers of GaAs and Al.sub.x Ga.sub.1-x As each 1700 Angstroms thick and numerous patents by Cho (e.g., U.S. Pat. No. 3,915,765) which state that MBE can be utilized to grow layers having a wide range of thicknesses; for example, 3 Angstroms (a single monolayer) to 100,000 Angstroms. But single monolayer growth in the Cho patents deals only with a single, isolated layer grown on a substrate. In view of the non-equilibrium nature of the MBE crystal growth process, it is not at all obvious that contiguous monolayers of different materials can be deposited with long range ordering; i.e., over distances large relative to the layer thickness. Nor is it clear what physical phenomena, such as inter-layer diffusion or surface-roughening, will inhibit or even prevent such ordering and under what growth conditions, if any, the adverse effects of such phenomena can be alleviated and ordering achieved.